5 Les sons impalpables du rêve (The impalpable sounds of a dream)
Polymodal, consisting of a blue-orange mode with a chordal ostinato
and cascades of chords, and a violet-purple mode having a copper
timbre. Note the pianistic writing, composed of triple notes, rapid
passages in chords, canon in contrary motion, hand crossing, various
staccatos, brassy louré, gem effects.

6 Cloches d'angoisses et larmes d'adieu (Bells of anguish and tears of
farewell) The bells combine several different modes: the "hum" (deep
bass) and the upper harmonies of the bells sound with luminous
vibrations. The farewell is purple, orange, violet.

8 Un reflet dans le vent (A reflection in the wind) The small storm
which opens and concludes the piece alternates veins of orange, and
green with black stains. The central development section is more
luminous. The second theme, very melodious, and wrapped in sinuous
arpeggios, is blue-orange in its first occurrence, and green-orange in
its second one. Violet, orange and purple dominate the entire piece.

Were these associations random or how did he determine the colors for each prelude? The only method I can think of would have something to do with Synesthesia.

Yes, Messiaen had consistent and strong synesthesia - read any article about him, really. That makes parts of his works hard to follow in the way that he experienced them. (Although to be honest, the same could be said for anyone who isn't a mystic practicing Catholic with an obsession for birds.)
– Kilian FothNov 15 '17 at 7:41

1 Answer
1

As Kilian Foth says in the comments, Messiaen famously has synesthesia. But his seemed to be more specific than most; whereas some composers may have said "this chord is blue," Messiaen would occasionally say something like "this chord is blue, but speckled with yellow and with a tinge of lilac." (This is a made up quote, but you get the idea.) You see this in some of the descriptions you listed, but those descriptions presumably describe entire preludes, not just individual sonorities.

Keep in mind, too, that synesthesia is not consistent from composer to composer; what one composer hears as blue, another may hear as green. (This is especially true with Scriabin.) Because of this, any performance directions you take from Messiaen's descriptions should probably be pretty limited. From an interpretive standpoint, you'd have more success focusing on the programmatic information he gave. In other words, "a reflection in the wind" likely has a greater impact on one's interpretation than does "violet, orange, and purple dominate the entire piece."

Just wanted to add that I believe the consensus is that Scriabin did not in fact have synesthesia, but rather believed philosophically in the relationship between music and color
– lightningAug 25 '18 at 1:55